PROJECT SUMMARY/ABSTRACT Acute myeloid leukemia (AML) comprises a genetically and clinically heterogeneous group of aggressive hematological malignancies. Despite advances in molecular characterization of AML, the majority of patients will relapse and die of their disease. In AML, oxidative phosphorylation (OxPhos) generates intracellular energy and metabolic intermediates necessary to promote growth and support survival. Unlike normal hematopoietic stem cells, AML and leukemia stem cells (LCS) overexpress anti- apoptotic mitochondrial protein Bcl-2, rely on OxPhos and are unable to utilize glycolysis when mitochondrial respiration is inhibited, indicating that the maintenance of mitochondrial function is essential for AML survival. We have identified a novel potent nanomolar inhibitor of OxPhos (OxPhosi) IACS-010759, selected from the series of more than 1,000 compounds across distinct structural classes. IACS-010759 has been found to inhibit complex I of OxPhos respiratory chain and block oxygen consumption. Our data demonstrated profound growth-inhibitory and pro-apoptotic effects of this agent in AML cell lines and primary AML cells at low nM concentrations, with minimal toxicity against normal BM cells. In turn, combination of OxPhos inhibitors and Bcl-2 inhibitor venetoclax is synergistic in AML. Daily dosing of IACS-010759 was well tolerated in mice, demonstrated strong efficacy in the in vivo xenograft studies utilizing the human AML patient-derived xenografts (PDX) and reduced phenotypically defined LSC fractions measured by novel technique of mass cytometry, CyTOF. Administration of OxPhosi following standard chemotherapy extended survival in primary AML PDX model. A Phase I clinical trial of IACS- 010759 in relapsed/refractory AML was recently launched at MDACC. We propose to test the hypothesis that OxPhos inhibition constitutes a novel therapeutic approach that targets a unique metabolic vulnerability of AML; and that combined blockade of mitochondrial respiration by OxPhos and Bcl-2 inhibitors will eliminate leukemia-initiating cells and produce objective responses. We will establish biomarkers of response to OxPhosi in vitro including RNA and metabolomics signatures, in a large series of primary AML with known genetic profiling, and validate these in the in vivo AML PDX models. We will further determine mechanisms of synergistic AML cell death when OxPhos inhibition is primed by Bcl-2 blockade with venetoclax, and characterize anti-AML and anti-LSC efficacy of such combination. We will further metabolically profile AML cells surviving standard chemotherapy, and test the hypothesis that OxPhosi will reduce or eliminate residual surviving AML cells. These concepts will be translated into Phase 1/2 study of standard chemotherapy and of Bcl- 2 inhibitor Venetoclax combined with IACS-010759 in patients with relapsed/refractory AML.